In hydraulic radial piston pumps employing the piston as a valve to open and close an intake port in the cylinder wall, hydraulic shock can occur as the piston moving at high speed covers the intake port. Furthermore, the piston and cylinder bore dimensions must be held closely to effectively seal this port. And piston rings are not recommended as they would be subjected to excessive wear as they cross the port. Moreover, displacement is lost as no compression can be attained while the intake port is open.